The action of phenytoin and valproate on thalamocortical responses was studied in adult rats. Single responses were not influenced by either drug. Paired-pulse potentiation of the initial components (first positive and first negative) observed with intervals from 50 to 200 ms under control conditions was abolished by phenytoin (60 mg/kg i.p.) but only moderately influenced by valproate (400 mg/kg i.p.). Paired-pulse potentiation of thalamocortical phenomena cannot be put into connection with the generation of the spike-and-wave rhythm.